<t>Why not diffusing directly the ZC bond ? Use the fact that P(t,T_i,T_i + 3m) is a martingale under the forward risk-neutral measure associated to T_i and that P(0,T_i,T_i + 3m)=P(0,T_i + 3m)/P(0,T_i). Then using P(T_i,T_i,T_i + 3m) = P(T_i,T_i + 3m) you can get the corresponding Libor fwd.Moreove...